This is a pretty good rundown on what the various ratings on windows such as U-factor, SHGC, air infiltration mean, and how they are calculated. (builditsolar.com)
For a good rundown on the Daniels vs. Zinn flap, see Peter Wood's account at the Chronicle of Higher Education. (firstthings.com)
(This article gives a good rundown of the confusing intricacies of the allocation order.) (wakingthered.com)